Repairing historical stone foundations
Many historic homes built before the 1920s have glorious stone foundations. While exceptionally durable and low maintenance, over time they do need to be repaired. The process of removing and repairing old mortar is called "repointing."
Why you can't use cement to repair stone
Repointing is not difficult, but if not done right, it can lead to damage to the foundation. This is where historical context is important. Over the years, many homeowners lacking the knowledge and understanding of traditional stone work attempted repairs by using cement that was inappropriate for the task. Cement often has a different strength than the stone and often doesn't expand and contract without damaging the wall. When too strong, cement can actually weaken the wall during frost and thaw cycles in colder regions. In some cases, the mortar should actually sacrifice itself to save the wall. Mortar is easy to repair and should be strong, but willing to give when necessary.
Portland cement can actually trap moisture inside the wall and encourage cracking. The key to a successful repair is finding an appropriate match for your mortar. Many regions have preservation experts that you can hire to consult to ensure a repair that will look great for another 100 years. Call your local chapter of Architectural Institute of America (AIA) or your local historic preservation office for a referral. It is crucial that your mortar is matched appropriately for the original type of stone. Importantly, masonry cement that is typically found in hardware stores can contain high percentages of Portland cement. It is NOT recommended for use on historic building stone work.
Guidelines for mortar matching
The National Trust for Historic Preservation says that the exact chemical makeup of the mortar is not vital as long as it matches these criteria:
- The new mortar must match the historic mortar in color, texture, and tooling. (If a laboratory analysis is undertaken, it may be possible to match the binder components and their proportions with the historic mortar, if those materials are available.)
- The sand must match the sand in the historic mortar. (The color and texture of the new mortar will usually fall into place if the sand is matched successfully.)
- The new mortar must have greater vapor permeability and be softer (measured in compressive strength) than the masonry units.
- The new mortar must be as vapor permeable and as soft or softer (measured in compressive strength) than the historic mortar. (Softness or hardness is not necessarily an indication of permeability; old, hard lime mortars can still retain high permeability.)
Mortar used on historic buildings is typically custom mixed. With proper consultation and research you can do this yourself. However, if there is a professional in your area it may be worth the extra cost. The National Trust suggests asking for contractors with at least 5 years experience.
How to repoint
Here is the basic process of repointing:
- Remove old mortar: Remove all loose mortar to a minimum depth of 2 - 2 1/2 times the width of the joints. If the joint is too shallow, the new mortar will not adhere and could pop out after curing.
- Mix the mortar: Mortar should be mixed consistently and evenly to match the older mortar.
- Fill the joint: Fill the deepest areas first. Then go back and fill shallower.
- Allow to cure: Local humidity and temperature levels will dictate how quickly the mortar will dry out. If it drys too quickly it can chalk. In this case it's encouraged to periodically mist the curing mortar with water.
- Age the mortar: The new mortar is mixed to match the original condition of the original mortar. The older mortar will looked aged and have a distinctly different color. You can either clean the old mortar or color or stain the new mortar.
For a deeper dive and more details on repointing historic stone work, check out Preservation Brief #2 from the National Trust for Historic Preservation to learn more.